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Old English steorbord, literally "side on which a vessel was steered," from steor- "rudder, steering paddle" (see steer (v.)) + bord "ship's side" (see board (n.2)). Cf. Old Norse stjornborði, Low German stürbord, German Steuerbord.
Early Germanic peoples' boats were propelled and steered by a paddle on the right side. French tribord (Old French estribord), Italian stribordo are Germanic loan-words.